Steels used in applications such as the manufacturing of home appliances and automobiles are required to have properties such as corrosion resistance, anti-aging properties, and formability.
The term “formability” is used herein to denote the ability of a material to undergo deformation into a desired shape without fracturing, tearing-off, necking, or shape errors such as wrinkling, spring-back, or galling occurring. In engineering, formability may be classified according to deformation modes. Examples of deformation modes include four machining modes: drawing, stretching, bending, and stretch-flanging.
Among the machining modes, stretching is simple, compared to deep-drawing, because a raw material almost never moves along an interface between the raw material and a die during stretching. In addition, stretching is known as a machining mode closely related to the elongation properties (elongation) of a material and is little affected by die conditions, unlike drawing, which is significantly affected by die conditions.
In a drawing-die process related to deep drawability, a material (plate) is placed on a drawing die and pressed using a blank holder, and then a punch is pushed into a recess of the drawing die to deform the plate. Therefore, the diameter of the plate is reduced after the drawing-die process. It is known that drawing is significantly related to the Lankford value (r-value), the ratio of strain in the thickness direction of a material to strain in the width direction of the material.
Particularly, the average plastic strain ratio (r-bar value) expressed by Formula 1 below and the plastic anisotropy (Δr value) expressed by Formula 2 below, obtained from r-values measured in different directions with respect to a rolling direction, are representative material properties describing drawability.r-bar=(r0+r90+2r45)/4  (1)Δr=(r0+r90−2r45)/2  (2)
where ri refers to the r-value of a specimen taken at an angle of i° from the direction of rolling.
As the r-bar of a material expressed by Formula 1 increases, the depth of a cup to be formed using the material may be increased, and thus it is considered that a high r-value guarantees a high degree of deep drawability.
In addition, planar anisotropy, an important quality property in a cup forming process, refers to the extent that the physical/mechanical properties of a material are dependent on direction. Planar anisotropy is basically caused by the strong directivity of each grain undergoing deformation such as plastic deformation. If grains are randomly distributed in a forming process, the grains may not have directivity, and thus the planar anisotropy of the grains may be low.
In general, however, grains in steel sheets have high directivity and thus exhibit plastic anisotropic behavior during a forming process. In a cup forming process, high planar anisotropy increases the occurrence of earing, which leads to height variations of formed portions of cups, thereby increasing defective products and material loss. If the Δr value, being an index of planar anisotropy, is close to 0, strain is uniform in all directions, and thus isotropic properties are present. Therefore, it is necessary to properly maintain the Δr value during a drawing process.
In the related art, as a method of guaranteeing the anti-aging properties and workability of steel, medium-low carbon Al-killed steel may be subjected to a hot-rolling process and a cold-rolling process, and then to a batch annealing process so as to efficiently adjust the contents of carbon and nitrogen dissolved in the steel.
However, the method requires a relatively long heat treatment time, resulting in low productivity. In addition, due to non-uniform heating and cooling patterns, material property variations increase in coils of steel sheets.
Therefore, according to a method proposed to remove the above-mentioned problems from ultra low carbon steel used as a material for a forming process and having anti-aging properties through a continuous annealing process, carbonitride forming elements such as titanium (Ti) or niobium (Nb) are added to the ultra low carbon steel so as to precipitate solute elements and obtain intended properties.
However, this method increases material costs and lowers the surface properties of steel due to the addition of relatively expensive elements. Furthermore, although such elements are added during a steel making process, it may be difficult to ensure workability such as cupping properties, due to the formation of disordered texture in a hot-rolling process.
Therefore, for example, hot-rolled steel sheets are used as a material for a forming process after a cold-rolling process and an annealing process are performed on the hot-rolled steel sheets to form an intended recrystallized texture in the steel sheets. In this case, however, material costs are also high because of the addition of alloying elements, and processing costs may be high because additional processes are necessary.
Therefore, there has been increasing interest in techniques for guaranteeing properties of hot-rolled steel sheets used as a material for a forming process, and in manufacturing methods using the hot-rolled steel sheets, so as to decrease manufacturing costs and the number of processes.
Related Patent Document 1 discloses a method of manufacturing a very thin hot-rolled steel sheet for a forming process using an endless processing technique by adding small amounts of manganese (Mn) and boron (B) to 0.01% to 0.08% carbon steel to decrease the Ar3 transformation point of the steel, reheating the steel to 1150° C., and performing a primarily coiling process at a temperature equal to or higher than the Ar3 transformation point, a joining process, and a final coiling process at a temperature of 500° C. or higher. According to the disclosed method, although the stretchability of the hot-rolled steel sheet is guaranteed because the hot-rolled steel sheet has an elongation of 45% or greater, the drawability of the hot-rolled steel sheet is not improved.
In addition, Patent Document 2 discloses a technique for ensuring drawability through the effect of self-annealing. According to the disclosed technique, ultra low carbon steel containing titanium (Ti) and/or niobium (Nb) is subjected to an endless hot-rolling process including a finish hot-rolling process in a ferrite single phase region, and the process temperature difference between the finish hot-rolling process and a coiling process is maintained to be 100° C. or less. However, according to the disclosed technique, relatively expensive alloying elements such as niobium (Nb) may be added to fix elements dissolved in steel, and it may be difficult to stably produce products because it is necessary to strictly manage the temperature of the finish hot-rolling process and the temperature of the coiling process for guaranteeing the formation of recrystallized grains.
(Patent Document 1) Japanese Patent Application Laid-open Publication No. H9-227950
(Patent Document 2) Japanese Patent Application Laid-open Publication No. H2-141529